This invention relates to class D amplifiers.
The use of semiconductors such as transistors in deflection current amplifiers in television receivers has resulted in cost saving and increased operating efficiency. In vertical deflection amplifiers relatively high efficiency class B output stages have been utilized to reduce the power dissipation across the transistors.
Even more efficiency can be achieved by utilizing a class D output stage in which the two output transistors are alternately turned on and off and function as switches. With such an arrangement the power dissipated in the output stages is primarily saturation and switching loss since the transistors are either cutoff or in saturation. The desired scanning current required by the vertical deflection coils coupled to an output terminal at the junction of the two transistors is essentially a sawtooth waveform. In order to operate the vertical deflection amplifier in a class D mode, a sawtooth waveform at the vertical deflection rate may be utilized to pulse width modulate a train of waveforms at the horizontal deflection rate. The modulated horizontal rate waveforms then serve as the switching control waveforms for the class D output stage. U.S. Pat. No. 3,456,150 discloses an arrangement for performing this type modulation which results in an average sawtooth current at the vertical deflection rate to be driven through the vertical deflection coils.
Since deflection yoke current must flow in the same direction in the yoke during both portions of each horizontal rate cycle to maintain the average desired net yoke current, the arrangement in that patent provides a filter circuit for the horizontal rate components including energy storage means which must be supplied during only one portion of each horizontal cycle. Therefore, the peak current conducted by the output stage is about twice the peak yoke current requirement. Additionally, the filter circuit required to remove the horizontal rate component from the deflection yoke and to maintain the desired yoke current flow in the same direction itself dissipates substantial power. Therefore, the combined peak current of the storage circuit, filter network and yoke greatly exceed the peak yoke current, thus requiring higher current capability in the output stage transistors and also undesirably increasing the total power consumed by the deflection circuit. Because this circuit operates class D, the output transistors dissipate less power than their counterparts in a class B output stage, but the overall power requirements of the total circuit may exceed those of a class B circuit.
In accordance with the invention, a class D amplifier is provided including a current amplifying stage comprising first and second serially coupled active current conducting devices. First and second unidirectional conducting devices are respectively coupled in parallel with the first and second active devices and are poled to conduct forward current in opposite directions than the active devices. A junction of the first and second active devices and unidirectional conducting devices forms an output terminal adapted for supplying current to a load. Means are coupled to the control electrodes of the active devices for providing a signal for alternately enabling the first and second active devices for conduction such that during an amplifying cycle the first active device conducts in the forward direction and the second active device conducts in the reverse direction, then the second active device conducts in the forward direction and the first active device conducts in the reverse direction for minimizing crossover distortion in the amplifier.